This invention relates generally to measuring and testing apparatus. More particularly, this invention relates to an ebulliometric technique for the determination of boiling points of liquids.
The boiling temperature of a liquid substance may be defined, as for example, in Andrews, D. H. and Kokes, R. J., Fundamental Chemistry, New York, John Wylie and Sons, 1963, p. 296, as the temperature where the pressure of the vapor phase of the substance is equal to the external pressure upon the liquid phase at the same temperature. The relationship between pressure and temperature at the boiling point of the substance, or more precisely on the boiling curve on a pressure-temperature diagram for the substance, may be determined by measuring the vapor pressure as a function of the temperature of the liquid. A prior art method for carrying out such a measurement is described in Moore, W. J., Physical Chemistry, Englewood Cliffs, N.J., Prentice-Hall, 1962, p. 106.
Moore discusses the use of an isoteniscope, shown schematically in FIG. 1. A bulb 1 and an attached short U-tube 2 are filled with a liquid sample to be studied. The bulb and U-tube are contained in a thermostat 3 having a thermometer 4 for measuring the temperature in the thermostat. Pressure means 5 may be provided for varying the pressure on the arm of the U-tube 2 external to the bulb 1. A manometer 6 measures the applied pressure on the external arm of the U-tube 2. The liquid is allowed to boil vigorously until all air is removed from the sample side of the U-tube 2. The temperature of the thermostat 3 is then varied over the range of temperatures at which the boiling point is to be determined, and the pressure applied by the pressure means 5 are adjusted until the arms of the U-tube contain the same height of liquid. The pressure and temperature as measured by the manometer 6 and the thermometer 16 are then recorded as a point on the pressure versus temperature boiling curve of the sample.
A difficulty with the isoteniscope method is related to the fact that there is a range of temperatures, at a given pressure, in which the vapor and liquid phases may simultaneously exist because of supersaturation and superheating phenomena. As illustrated in FIG. 2, taken from Cole, R., "Homogeneous and Heterogeneous Nucleation," in: Van Stralen, S., and Cole, R., Boiling Phenomena, (New York, McGraw-Hill, 1979), p. 71, boiling is characterized by an envelope rather than by a single curve on a pressure versus specific volume (or, equivalently, temperature) diagram. At any given pressure below the critical point there is a range of specific volumes of the fluid and vapor phases possible for the system. Fluid between the dashed line passing through the point C and the solid line passing through the point B may exist in a metastable superheated state, and vapor to the right of the dashed line passing through the point E and the solid line passing through the point F may exist in a metastable supersaturated state. A liquid-vapor mixture in the thermodynamic region between the two dashed lines would be mechanically unstable.
In the isoteniscope method the determination of vapor pressure is complicated by the fact that the vapor on the sample side of the U-tube may be supersaturated if the measurements of vapor pressure are made after cooling. On the other hand, if measurements are made after heating of the sample from a lower temperature then the liquid may be superheated. In either case, the relationship between pressure and temperature will not be accurately given, thereby leading to inaccurate determination of the boiling point curve.
The problem in the isoteniscope method is illustrative of problems in prior art methods of determining boiling points where bulk measurements are made on material with both the liquid and vapor phases present. When the equilibrium point at which the measurement is to be made is approached from a lower temperature, then the liquid will generally be superheated. When the equilibrium point is approached from a higher temperature, then the vapor will generally be supersaturated.
A second difficulty with prior art methods involving bulk fluid phase and gas phase materials simultaneously present is that bulky and complicated apparatus are needed, and such methods do not lend themselves easily to automation.